1. Field of the Invention
The present invention relates to a light-emitting device that uses a luminous organic film. Further, the present invention relates to electric equipment using the light-emitting device as a display portion or a light source. It is to be noted that the luminous organic film, which can be used in the present invention, includes all organic films that emit light (fluorescent light and/or phosphorescent light) via either a singlet excitation or a triplet excitation, or via both excitations.
2. Description of the Related Art
In recent years, development is proceeding in a light-emitting device (hereinafter referred to as an EL light-emitting device) employing a luminous element (hereinafter referred to as an EL element) that uses a luminous organic film (hereinafter referred to as an organic EL film) that provides EL (Electro Luminescence). The EL light-emitting device has an EL element that is composed of an anode, a cathode, and an organic EL film sandwiched therebetween. The emission of light can be attained by applying a voltage between the anode and the cathode.
At this point, a hole from the anode is injected into the EL material, and an electron from the cathode is injected therein. Electric charges (carriers) injected from both the electrodes move in the interior of the organic EL film to thereby re-couple. An excitation state is generated by the re-coupling of the carriers, and a portion thereof is converted into photons. Luminescence can be made visible by extracting these photons to the outside.
Such a conventional light-emitting mechanism of the EL element is shown in FIGS. 2A and 2B. Shown in FIG. 2A is the conventional junction structure of the EL element in which reference symbol 201 denotes a cathode, reference symbol 202 denotes an electron transfer layer, reference symbol 203 denotes an emission layer, reference symbol 204 denotes a hole transfer layer, and reference symbol 205 denotes an anode. Further, shown in FIG. 2B is the carrier injection process thereof. A voltage is applied between the cathode 201 and the anode 205 to thereby inject an electron 206 and a hole 207. The injected electron 206 and hole 207 re-couple, whereby an emission 208 is attained.
Taking into consideration such a light-emitting mechanism, the efficiency of light emitted from the EL element, that is, the emission efficiency (expressed as xcex7 (emission)) is expressed as the following equation.
xcex7(emission)=xcex7(injection)xc3x97xcex7(re-coupling)xc3x97xcex7(excitation)xc3x97xcex7(quantum)
Here in this equation, xcex7 (injection) denotes the efficiency when the carrier is injected from the electrode, xcex7 (re-coupling) denotes the re-coupling efficiency of the electron and the hole, xcex7 (excitation) denotes the efficiency of generating a singlet exciton due to the re-coupling, and xcex7 (quantum) denotes the efficiency of converting the singlet exciton to a photon.
The xcex7 (injection) efficiency originates in an electric potential barrier in the interface between the cathode (or the anode) and the EL material, and changes. The lower the electric potential barrier, the higher the xcex7 (injection) efficiency is. The xcex7 (re-coupling) efficiency changes due to the injection balance of the carrier (balance of the ratio of the injected electron and hole), and is influenced by the carrier transfer characteristic of the emission layer (the organic EL film that will actually emit light). Further, the xcex7 (excitation) efficiency is the generating efficiency of the singlet exciton that contributes to the emission of light, and is theoretically set (fixed) at about 0.25. Further, the change of the xcex7 (quantum) efficiency depends on whether the emission layer is crystalline or non-crystalline. Generally speaking, a higher value can be attained from a crystalline emission layer than from a non-crystalline one.
In addition, until the photons, which are generated in the emission layer, are extracted to the outside, most of them are lost (about 80% are lost) due to diffusion and thermal deactivation. Therefore, light that is actually observed includes the loss of the photons. Thus, in the light-emitting mechanism process of the EL element, the emission efficiency is reduced due to various factors. In order to obtain high emission efficiency, the above-mentioned various efficiencies have to be raised to thereby attain a total high emission efficiency.
The present invention has been made in view of the above, and therefore has an object to provide a method of raising a re-coupling efficiency of carriers in an EL element to thereby provide a light-emitting device having high emission efficiency. Further, another object of the present invention is to provide electric equipment using the light-emitting device as a display portion or a light source.
In the present invention, attention is paid to the band structure of the EL element in order to improve the re-coupling efficiency (expressed as xcex7 (re-coupling)) of an electron and a hole, and it is characterized in that the probability of the re-coupling is increased by enclosing the electron and the hole in a specific region to thereby enhance the re-coupling efficiency thereof. Therefore, the EL element having the band structure shown in FIG. 1A is formed.
In FIG. 1A, reference symbol 101 denotes a cathode, reference symbol 102 denotes an electron transfer layer, reference symbol 103 denotes an emission layer, reference symbol 104 denotes a hole transfer layer, and reference symbol 105 denotes an anode. Further, an electron trap region 106 and a hole trap region 107 are formed in the interior of the emission layer 103. It is to be noted that the structure of the EL element may be a structure provided with either the electron trap region 106 or the hole trap region 107.
The electron trap region 106 here is a region that has the action of enclosing within the emission layer an electron that is transferred at the lowest unoccupied molecular orbit (LUMO) level of the emission layer 103. In addition, the electron trap region 106 denotes a region that indicates a LUMO level that is lower than the LUMO level of the emission layer 103. The hole trap region 107 is a region that has the action of enclosing within the emission layer a hole that is transferred at the highest occupied molecular orbit (HOMO) level of the emission layer 103, and denotes a region that indicates a HOMO level that is higher than the HOMO level of the emission layer 103.
The electron trap region 106 can be formed by constructing a structure in which an organic film or a cluster of organic substances, which has the effect of lowering the LUMO level, is sandwiched between the emission layer 103. Further, the hole trap region 107 can be formed by constructing a structure in which an organic film or a cluster of organic substances, which has the effect of raising the HOMO level, is sandwiched between the emission layer 103.
Simultaneously with the provision of the above electron trap region 106 or the hole trap region 107, a hole prevention layer may be provided between the electron transfer layer 102 and the emission layer 103, or an electron prevention layer may be provided between the emission layer 103 and the hole transfer layer 104. Of course, the structure thereof may be a structure that is provided with both the hole prevention layer and the electron prevention layer.
If the band structure of the EL element shown in FIG. 1A is applied, then the carrier injection process thereof is a process as shown in FIG. 1B. In other words, an electron 108 transferred at the LUMO level is enclosed in the electron trap region 106 that is provided in the interior of the emission layer 103. On the other hand, a hole 109 transferred at the HOMO level is enclosed in the hole trap region 107. As a result, the re-coupling of the electron 108 and the hole 109 occurs between the electron trap region 106 and the hole trap region 107, whereby the emission of light can be obtained.
At this point, in the present invention, because re-coupling takes place under the, state where either the electron or the hole is enclosed in the trap region, the re-coupling efficiency (xcex7 (re-coupling)) can be improved more in comparison with a conventional one. It can be said that the concept of forming a level well in the interior of the emission layer to enclose carriers is an unprecedented concept.
Thus, among some parameters which contributes to the emission efficiency, the efficiency (xcex7 (re-coupling)) that originates from the re-coupling of carriers can be improved by implementing the present invention, whereby the emission efficiency of the EL element is enhanced. Therefore, the driving voltage of the EL element can be set at a low rate even for attaining a luminance equivalent to that of the prior art, so that the consumption power of the light-emitting device can be reduced. In addition, the lowering of the driving voltage leads to the suppression of the deterioration of the organic EL film, resulting in the enhancement of the reliability of the light-emitting device.